This container can be pulled via:
  docker pull registry.opensuse.org/filesystems/ceph/octopus/images/ceph/prometheus-alertmanager:0.21.0
  docker pull registry.opensuse.org/filesystems/ceph/octopus/images/ceph/prometheus-alertmanager:0.21.0.6.2
  docker pull registry.opensuse.org/filesystems/ceph/octopus/images/ceph/prometheus-alertmanager:latest
  docker pull registry.opensuse.org/filesystems/ceph/octopus/images/ceph/prometheus-alertmanager:leap15.2

Set DOCKER_CONTENT_TRUST=1 to enable image tag verification.